Tungsten plugs transfer current between upper and lower interconnect levels in a semiconductor integrated circuit. Holes are formed in an insulating layer by dry etching. The tungsten is deposited in the holes by a chemical vapor deposition (CVD) process. Being formed via a surface driven chemical process, the tungsten grows from the bottom and walls of the holes. Growth of the tungsten stops when the holes are closed. The holes usually do not close perfectly because of growing layer surface roughness. Because the holes are not perfectly filled, there is usually a seam in the center of the tungsten plug. After tungsten deposition, an etch or chemical mechanical polishing (CMP) step removes the overburden. Both dry etch and CMP involve significant degree of chemical reaction with the tungsten overburden. This reaction is active on all kinds of interfaces. Usually the seam becomes the most chemically vulnerable plug location, which is etched out during CMP or dry etch. Often one finds a keyhole in the center of the plug as a result of the chemical etching. The holes affect the plug electrical properties and electromigration performance and also trap chemicals inducing corrosion effects. Therefore it is important to evaluate the deposition quality of the tungsten plugs.
The deposition processes parameters may be adjusted to optimize the quality of the plugs. However, to do so it is necessary to determine how many plugs have unacceptably large voids. This typically requires etching away the tungsten until the voids in the plugs become visible. A technique is known as electron beam activated chemical etch (EBACE). In this technique an etchant is introduced through a needle into the field of view of a scanning electron microscope proximate the surface of an integrated circuit device. The etchant is usually one that is known to respond to electron-beam induced activation. Thus, localized etching of the surface can be combined with real time imaging of the surface as it is etched. Unfortunately, this technique typically involves sacrificing some of the devices on a particular wafer, which reduces yield and increases costs.
Thus, there is a need in the art for a method of determining the quality of tungsten deposition that overcomes the above disadvantages.